This invention relates to a circuit for handling power dissipation through telephone subscriber line power supply interface devices, and particularly to monolithically integratable telephone interface devices and to systems which incorporate them.
Various devices have been proposed for limiting the power dissipated in telephone circuits, to limit (e.g.) the heating of their components. One of these comprises circuitry for limiting the maximum current which can be supplied to the line, in combination with a thermal protection circuit which limits the maximum temperature attainable within the circuit connected to the load. However, although this device functions properly during normal operation of the telephone circuit, it cannot operate effectively when the load is subjected to a longitudinal (i.e. common-mode) current. When the load absorbs a common-mode current from the feed line, devices of the aforesaid type are no longer able to perform their function because the operating threshold of the limiting means is usually higher than the current passing through the telephone in this situation. However, the thermal protection circuit continues to operate, and when it does it changes the operating characteristics of the protected circuit to the extent that its operation becomes unacceptable.
One approach to overcome these drawbacks has been to couple the load-connected circuit to an external component within which part of the power absorbed by the circuit is dissipated. Hence the power dissipated within the circuit is controlled indirectly by controlling the power dissipated within the external component. This method enables good results to be obtained during normal operation of the telephone, but is problematic when the load to which the telephone circuit is connected becomes the center of a common-mode current.
This is because the known devices again in this case suffer from the aforesaid drawbacks, and in particular do not enable the temperature increase in the external component (caused by the increase in the power dissipated within this component) to be limited.
The incorporation of such devices into telephone exchanges of ever more compact design requires that new designs be evolved for circuits and systems to provide improved power distribution and active power control under any operating conditions. Power distribution is performed by using an auxiliary element (e.g. an external transistor), connected to each interface device, which is effective to dissipate some of the power.
Under certain exceptionally severe conditions of the line, and in the presence of longitudinal currents, the dissipated power may fall outside the upper admissible limits by standard packages even in the presence of an external auxiliary element adapted to dissipate some of the power. Such longitudinal currents may be caused by one or both of the line wires being temporarily connected to ground ("ground key" situation), or by failures in the line ("fault" situation).
As known to persons skilled in the art, an electronic interface circuit between a subscriber's telephone line and exchange control components in general comprises a circuit structure of the bridge type formed by two output amplifier components between which the subscriber's telephone line, and all the apparatus connected thereto, is inserted as a load.
These amplifier components drive the line in phase opposition when signals are present.
The "transverse" line current I.sub.T is the sum of the direct current supplying the line and the signal current which is generally of an alternating type. This transverse current I.sub.T is of identical intensity in the two wires of the line, but has opposite directions of flow.
However, if a two-wire telephone line (or more generally a two-wire transmission line) is in the vicinity of electrical lines having an alternating current at power frequencies (or at industrial-use frequencies), or is in the vicinity of other telephone lines in which high intensity signals, such as ringing signals, are being transmitted, such nearby currents may induce "longitudinal" or "common mode" currents I.sub.CM in both wires of the telephone line. Such currents will often have identical intensities and directions of flow in both wires of the line.
These common mode currents I.sub.CM may, in the cases discussed above, be of an alternating type, but do not, in general, have a waveshape which is fixed over time. Thus, when such induced common mode currents are superimposed on the transverse line current I.sub.T, they alter its value in an unpredictable way.
If the overall resultant currents in the wire of the line with the higher potential and the wire of the line with the lower potential are designated conventionally by I.sub.A and I.sub.B respectively, the following may be expressed: EQU I.sub.A =I.sub.T +I.sub.CM EQU I.sub.B =I.sub.T -I.sub.CM
in which the actual directions of each current obviously have to be borne in mind in accordance with known electrical conventions.
This shows that it is sufficient, in theory, to add and subtract the total currents I.sub.A and I.sub.B for an immediate "measurement" of the transverse line current I.sub.T and the longitudinal common mode currents I.sub.CM respectively. This gives: EQU .vertline.I.sub.A +I.sub.B .vertline.=2.vertline.I.sub.T .vertline. EQU .vertline.I.sub.A -I.sub.B .vertline.=2.vertline.I.sub.CM .vertline.
It is particularly necessary to obtain a measurement of the common mode currents I.sub.CM in cases in which these currents are intentionally induced in the line so that specific functions may be carried out (such as are used in private branch exchanges), e,g, to enable the transfer of incoming calls from one subscriber's set directly to another by means of the appropriate key on the set itself.
In reality, a circuit capable of carrying out these simple operations in all longitudinal and transverse current conditions is comparatively complex with the result that it is expensive, if monolithically integrated, both from the point of view of integration area occupation and design problems.
If the adding and subtracting operations on the overall currents in the line are to give representative measurement results under all operating conditions, it is necessary, in the first instance, to take into account the possibility of inversions of the line polarity, since it is this polarity which determines the direction of flow of the transverse current I.sub.T in the line.
In addition, the possibility, particularly in very long transmission lines, in which the transverse line current I.sub.T is obviously reduced, while the probability of longitudinally induced currents is higher, of the intensity of the longitudinal common mode currents I.sub.CM being greater than the intensity of the transverse line current I.sub.T, should not be neglected.
In this case, the overall currents in the line I.sub.A and I.sub.B have the same direction which is not determined by the line polarity, but is variable over time in a uniform manner with the direction of the common mode currents induced.
Since the active electronic components conduct essentially in a monodirectional manner during normal polarization and operating conditions, a circuit which may be monolithically integrated for measuring longitudinal and transverse line currents must be designed in such a way that it has an overall configuration which is compatible with input currents having any direction.
In telephone interface circuits, as is normally the case for all integrated circuits, there is a maximum instantaneous current limitation to avoid damage to the circuit components (the current limit may have a typical value of 100 mA).
The part of the interface circuit which acts as a supply source for the line operates at high voltages (typically from -48 to -60 V with respect to the ground potential), and the electrical power dissipated is therefore very high.
Adequate limitation of the power dissipated must therefore be provided in the circuit by means of the heat protection conventionally incorporated in integrated circuits.
Several arrangements have been proposed for limiting the maximum current supplied to the line, and hence the amount of power dissipated. Such prior arrangements, while performing their expected function satisfactorily in normal operation of the telephone system, are unable to operate effectively when the line is subjected to a longitudinal or common mode current at a non-zero mean value. In fact, when the load draws a longitudinal current from the supply line, such limiting arrangements may fail to perform their function because their cut-in thresholds are defined with respect to transverse current and do not fully allow for the possible range of conditions in longitudinal current.
Some telephone systems incorporate a thermal protection device which operates, whenever the temperature reaches a predetermined upper threshold level, to change the operational characteristics of the protected system, and, consequently, to disrupt its normal operation.
A circuit structure which enables dissipated power to be controlled even under a condition of line unbalance is described in European Patent Application No. 91117504.0, which is hereby incorporated by reference. That Application discloses a technique whereby in an unbalanced condition of the line, the power dissipated by the subscriber line interface circuit and the external transistor is reduced by limiting the line transversal current. However, the circuit described there can only operate as expected when the transversal current limitation is able to limit the largest longitudinal current, which would only occur when the two line terminations operate in single-quadrant class B.
Additional discussion of subscriber Line Interface Circuits may be found in U.S. Pat. Nos. 5,138,658, 4,908,856, 4,709,388, 4,387,273, 4,381,427, all of which are hereby incorporated by reference.